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Old 07-10-2008, 05:45 PM   #2 (permalink)
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wheel house wind shear

Originally Posted by pasadena_commut View Post
Covered rear wheels help with the airflow along the body. I wonder what they do to airflow within the wheel well.

The boundary conditions for the moving air give it zero velocity relative to the spinning wheel at the surface of the wheel, and zero velocity relative to the inside surface of the wheel well (including the added wheel cover). However, the outside edge of the tire is spinning at the vehicle's speed relative to the car, which makes me wonder how much new drag is introduced by the airflow shear between the fixed wheel cover and the surface of the spinning wheel. The distance from the inside of the wheel to the wheel well wall parallel to it tends to be at least 6-8". Presumably that is far enough that drag from shear is minimized. However, rear wheel covers in these forums appear to be significantly closer than that, some appear to be only an inch or so from the wheel. Are these covers so close that the air shear between the wheel and the wheel cover produces measurable drag?

It would be good to know what the minimum separation must be in order to avoid inducing this sort of drag.
Pasadena,I got a little confused at to whether you were referring to wheel covers or wheel skirts,and I'm thinking you were referring to the latter.There is a body of research that's been published for wheel house drag.The tire tread acts as a crude turbine and there are "windage"loads associated with tires themselves.Also,the volume relationship, between the wheel house and the tire/wheel combination, has been shown to effect drag.No doubt,there is interference drag between the rotating wheel assemblyand the inner surface of the skirt,however,it has been demonstrated by numerous researchers,that the presence of the skirt,even in very close proximity to the tire/wheel,demonstrates a measurable drag reduction in spite of any internal interference drag,compared to the same vehicle without them.Also,aft-body modifications,such as the boat-tail are compromised in performance without the presence of such skirts.For a deeper look,you'll want to check out studies cited in Hucho's book,listed in the "sticky" aero thread here at the top of the aero forum page.All this drag adds 0.05 to the drag coefficient.Ford and Honda have gone after it with a vengeance with their concept and record cars.Its an area well worth the look!
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